In some vehicles, vacuum is used to operate or assist in the operation of various devices. For example, vacuum may be used to assist a driver applying vehicle brakes, turbocharger operation, fuel vapor purging, heating and ventilation system actuation, and driveline component actuation. If the vehicle does not produce vacuum naturally, such as from the intake manifold, then a separate vacuum source is required to operate such devices. While an aspirator or an ejector can produce vacuum when supplied with either boost or manifold vacuum, the depth of vacuum produced will be a function of the difference in pressure between the pressure applied to the motive port and the pressure applied to the discharge port. However, in boosted engines where intake manifold pressures are often at pressures greater than atmospheric pressure, intake manifold vacuum may be replaced or augmented with vacuum from an ejector. An ejector, as used herein, is a converging, diverging nozzle assembly connected to a pressure source above atmospheric pressure. By passing pressurized air through the ejector, a low pressure region may be created within the ejector so that air can be drawn from a vacuum reservoir or may directly act on a device requiring vacuum, thereby reducing pressure within the vacuum reservoir or device requiring vacuum.
Typical ejectors cannot produce a suction pressure below atmospheric pressure when the motive pressure exceeds 192 kPa absolute, and the maximum vacuum is produced with a motive pressure of less than 135 kPa absolute. However, boosted engines routinely operate at above 135 kPa absolutes, so there is a need to improve the performance of an ejector on such a vehicle. These conventional ejectors are limited in the vacuum they can produce, in part because the different boost pressures cause the location of minimum pressure to move to different locations inside the ejector. Specifically, as the motive pressure increases beyond a certain value relative to the discharge pressure, the point of minimum vacuum creation moves progressively down the discharge passage.
What is needed is an ejector that can produce vacuum over a range of motive pressures.